1. Field of Invention
The present invention relates to a time adjustment device that corrects the time based on signals from a positioning information satellite such as a GPS satellite, to a timepiece that has the time adjustment device, and to a time adjustment method.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time) with extremely high precision.
An electronic timepiece that adjusts the time using time information (GPS time) from GPS satellites is taught, for example, in Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251.
In the electronic timepiece that uses the GPS time, the reception unit that receives the signals from the GPS satellites must receive the TOW signal (Time of Week, GPS time information that restarts every week and denotes the number of seconds from the beginning of the week) contained in the GPS satellite signals in order to get the time information from the GPS satellite.
In order to shorten the time needed from the start of signal reception until the correct time can be output, this electronic timepiece prioritizes the satellite search sequence based on the past reception history, selects a positioning information satellite, and adjusts the time without determining its own position.
By prioritizing the order of the satellites selected for reception based on the past reception history to get the time information, the electronic timepiece taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251 is effective in an electronic timepiece that is located indoors and has a limited angle of exposure to the sky through a window.
However, if the electronic timepiece is a wristwatch and is worn on the wrist as the user moves or walks around, selecting the satellite based on the past reception history is substantially meaningless because of the following conditions.
First, because the person wearing the timepiece moves around, the location of the timepiece changes and the timepiece may therefore not be able to receive signals from the satellite from which signals were successfully received in the past (such as when a user in Japan takes the timepiece overseas).
Second, because the person wearing the timepiece moves around, the reception environment is not always the same (because signals may be blocked by homes, buildings, trees, or other objects).
Third, when the user walks around with the timepiece worn on the wrist, the orientation of the antenna is not constant and the positioning information satellites that are suitable for actual reception change even if the satellite signals are received at the same time of day.
Reliably receiving the satellite signals can therefore be difficult, and setting the correct time based on the acquired time information can be difficult, even if the technology taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251 is used in a wristwatch.